There are many reasons for Nick Clegg's growing unpopularity in Tory ranks. Once an asset to the Government - supporting deficit reduction and welfare reform, for example - he has become an increasing drag on the Coalition and the Conservative Party for at least three key reasons:

His personal unpopularity - YouGov's Peter Kellner called the Deputy PM "toxic" on today's World at One and this toxicity doesn't just hamper the Liberal Democrats but it hampers the whole Government. One Tory MP likens Mr Clegg to "a contagious disease that infects the whole governing enterprise... it's like the whole ship is weighed down by the sinking Lib Dem leader and unable to get up any speed". Every time he appears on TV or radio voters are reminded of his broken promise on tuition fees.

His internal weakness - no senior Tory I speak to expects Clegg to still be leader of his party by the time of the next election. The view is that only a miracle can restore a semblance of electability. His apology video (now watched by 2.2 million people on YouTube) - insofar as it was noticed at all - turned him into a laughing stock. Clegg's authority in his party is largely spent and that makes it difficult for him to get his colleagues to sign up to any serious reforms. There have been moments in 2012 - such as on the new GCSE or 'Gove-Level' - where the Coalition recovered some of its early rose garden promise but these moments are now exceptional rather than normal. It is far from clear that unity can be maintained in 2013 when long-delayed cuts to current rather than capital government programmes are due to be inked into the ledger.

His retoxification of the Tory brand - For the moment the Tory leadership indulges his constant attacks on the Conservative Party (last week it was welfare, this week it's Europe) - noting that (1) they are necessary to avoid an early leadership challenge from angry Lib Dems; (2) that they are not boosting Lib Dem fortunes; but (3) are playing an unhelpful role in retoxifying the Conservative Party. Most senior Conservatives hope that Clegg is replaced by a left-leaning Lib Dem like Tim Farron or Vince Cable before the next election. Only a Farron or Cable, they believe, can win back some of the left-wing votes that have heamorraghed from the Lib Dems to Labour and are producing the 'Gettleston Effect' which will see second-placed Labour candidates out poll incumbent Tory MPs without having to even lift a finger.

Pete Hoskin has offered an alternative view of Nick Clegg, describing him "one of the few evangelical public service reformers". Pete's is increasingly a minority view - even amongst Cameron's closest advisers after his boundary review U-turn.